1. Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting display, and more particularly, to an organic light emitting display and a method of compensating for a threshold voltage thereof.
2. Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (hereinafter, abbreviated as “OLEDs”) capable of emitting light. Such an active matrix organic light emitting display has advantages of a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element typically includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
The organic light emitting display arranges pixels, each including an OLED, in a matrix form, and adjusts a luminance of the pixels depending on a gray scale of video data. Each pixel typically includes a driving thin film transistor (TFT) for controlling a driving current flowing in the OLED. It is preferable that electrical characteristics (including a threshold voltage, mobility, etc.) of the driving TFT are equally designed in all of the pixels. However, in practice, the electrical characteristics of the driving TFTs of the pixels are not uniform due to various causes. A deviation between the electrical characteristics of the driving TFTs results in a luminance deviation between the pixels.
Various compensation methods of compensating for the threshold voltage of the driving TFT are known. FIGS. 1 and 2 show one of the various compensation methods. An external compensation method illustrated in FIGS. 1 and 2 operates a driving TFT DT in a source follower manner and senses a threshold voltage Vth of the driving TFT DT. The source follower manner determines a change in the threshold voltage Vth based on a sensing value input to an analog-to-digital converter (ADC). However, accurate sensing of the threshold voltage Vth of the driving TFT DT using the source follower manner has to be performed after the driving TFT DT is turned off and a drain-source current Ids of the driving TFT DT becomes zero. Therefore, a long time Tx is required to sense the threshold voltage Vth.
More specifically, a sensing data voltage Vdata greater than the threshold voltage Vth is applied to a gate electrode of the driving TFT DT, so as to sense the threshold voltage Vth. When an initialization voltage Vref is applied to a source electrode of the driving TFT DT, the driving TFT DT is turned on because a gate-source voltage Vgs of the driving TFT DT is greater than the threshold voltage Vth. In this instance, the drain-source current Ids of the driving TFT DT depends on a difference Vgs between a gate voltage Vg (VN1) of the driving TFT DT and a source voltage Vs (VN2) of the driving TFT DT. In an initial sensing period, in which the source voltage Vs (VN2) of the driving TFT DT starts to increase, because the gate-source voltage Vgs of the driving TFT DT is large, a channel resistance of the driving TFT DT is small. As a result, the drain-source current Ids of the driving TFT DT is large. However, as the source voltage Vs (VN2) of the driving TFT DT gradually increases, the gate-source voltage Vgs of the driving TFT DT decreases. Therefore, the channel resistance of the driving TFT DT increases. As a result, the drain-source current Ids of the driving TFT DT decreases. When the drain-source current Ids of the driving TFT DT decreases, a charge amount accumulated in a sensing capacitor Cx decreases. Therefore, a time required for the gate-source voltage Vgs of the driving TFT DT to become the threshold voltage Vth increases. As the sensing time of the threshold voltage Vth increases, the amount of time available for displaying an image (e.g., the image display time) is reduced. Thus, in order to increase the image display time, the sensing time of the threshold voltage Vth needs to be reduced.